390 THE ORIGIN OF VERTEBRATES 



metanephric tubules are formed, and the former alone gives origin 

 to a duct, which forms the basis for the generative and urinary 

 ducts, and is called the segmental duct. The mesonephric tubules, 

 called also the Wolffian body, open into this duct. 



In order to make the embryology of these excretory organs quite 

 clear, I will make use of van Wijhe's phraseology and also of his 

 illustrations. He terms the whole ccelomic cavity the proecelom, 

 which is divisible into a ventral unsegmented part, the body-cavity 

 or metaccelom, and a dorsal segmented part, the somite. This latter 

 part again is divided into a dorsal part — the cpimere — and a part 

 connecting the dorsal part with the body-cavity, to which therefore 

 he gives the name of mesomere. 



The cavity of the epimere disappears, and its walls form the muscle 

 and cutis plates of the body. The part which forms the muscles is 

 known as the myotome, which separates off from the mesomere, leaving 

 the latter as a blind sac — the mesocoelom — communicating by a narrow 

 passage with the body cavity or metaccelom. At the same time, from 

 the mesomere is formed the sclerotome, which gives rise to the skeletal 

 tissues of the vertebra?, etc., so that van Wijhe's epimere and mesomere 

 together correspond to the original term, pro to vertebra, or somite of 

 Balfour ; and when the myotome and sclerotome have separated 

 off, there is still left the intermediate cell-mass of Balfour and 

 Sedgwick, i.e. the sac-like mesoccele of van Wijhe, the walls of which 

 give origin to the mesonephrotome or mesonephros. Further, accord- 

 ing to van Wijhe, the dorsal part of the unsegmented metaccelom is 

 itself segmented, but not, as in the case of the mesoccele, with respect 

 to both splanchnopleuric and somatopleuric walls. The segmentation 

 is manifest only on the somatopleuric side, and consists of a distinct 

 series of hollow somatopleuric outgrowths, called by him hypomeres, 

 which give rise to the pronephros and the segmental duct. 



Van Wijhe considers that the whole metaccelom was originally 

 segmented, because in the lower vertebrates the segmentation reaches 

 further ventral-wards, so that in Selachia the body-cavity is almost 

 truly segmental. Also in the gill-region of Amphioxus the cavities 

 which are homologous with the body-cavity arise segmentally. 



As is well known, Balfour and Semper were led, from their 

 embryological researches, to compare the nephric organs of vertebrates 

 with those of annelids, and, indeed, the nature of the vertebrate 

 segmental excretory organs has always been the fact which has kept 



